J Physiol Volume 582, Number 2, 789-799, July 15, 2007 DOI: 10.1113/jphysiol.2007.127969

The up and down bobbing of human walking: a compromise between muscle work and efficiency

Firas Massaad¹, Thierry M. Lejeune¹ and Christine Detrembleur¹

¹ Rehabilitation and Physical Medicine Unit, Université catholique de Louvain, Avenue Mounier 53, 1200 Brussels, Belgium

Human walking has a peculiar straight-legged style. Consequently, the body's centre of mass (CM) moves up and down with each step, which is noticeable in their up and down head bobbing while walking. This vertical CM movement enables humans to save energy via a pendulum-like mechanism but is probably a relatively recent locomotor innovation insofar as earliest bipeds may have walked flexed and flat. We investigated the mechanics, energetics, muscle efficiency and optimization of human walking by decreasing and increasing the vertical CM displacement (flat and bouncy walking) in comparison to normal walking at six speeds (1–6 km h^–1). In both flat and bouncy walking, the pendular mechanism was reduced and the energy cost was increased. However, this increase was unexpectedly much sharper in flat walking where muscles provided normal mechanical work but with a decrease in muscle efficiency. In bouncy walking, muscles provided extra mechanical work in an efficient way. Our results showed that not only do humans bob up and down in normal walking to save energy via a pendulum-like mechanism but also to make their muscles work efficiently. Actually, walking flat makes the muscles work in unfavourable conditions that waste energy. Furthermore, we are still close to a flat CM displacement relative to our current ability to change this displacement, which suggests that reducing vertical CM displacement is indeed important but only to certain limits. Evolution may ultimately have chosen the best compromise between flat locomotion that requires little work to move and bouncy locomotion that improves muscle efficiency to minimize energy consumption.

(Received 10 January 2007; accepted after revision 20 April 2007; first published online 26 April 2007)
Corresponding author F. Massaad: Rehabilitation and Physical Medicine Unit, Université catholique de Louvain, Tour Pasteur 5375, Avenue Mounier 53, 1200 Brussels, Belgium. Email: firas.massaad@gmail.com

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